This application is similar in some respects to U.S. Ser. No. 109,956, now U.S. Pat. No. 4,321,311 (Columnar Grain Ceramic Thermal Barrier Coating) by T. Strangman filed on even date herewith.
The superalloy art has long sought to combine the properties of ceramics with the properties of metals. Thus for example, many attempts have been made to provide protective ceramic coatings to metal articles which find application at elevated temperatures so as to combine the thermal properties of ceramics with the ductility of metals.
The primary problem which has not been successfully solved heretofore is that the substantial difference in the coefficient of thermal expansion of metals and ceramics invariably leads to failure of ceramic coatings under conditions of severe thermal cycling.
One approach which has been employed in an effort to overcome this problem is that of grading the coating from essentially all metal at the metal surface to all ceramic at the outer surface of the coating. In this way it is believed that the coefficient of thermal expansion will change gradually through the coating thickness and the stress resulting from thermal cycling will not be sufficient to cause damage to the coating. Such an approach is described in U.S. Pat. No. 3,091,548 to Dillon. The problem with the graded approach is that the discrete metal particles in the graded coating oxidize and increase in volume producing unacceptable stresses in the coating.
In the general area of metal-ceramic combinations, it is known to use segmented ceramic pieces such as tiles which are bonded to metal structures for their protection. In this approach, which is generally applied to large articles, the segments are not bonded to each other, and the gaps between the tiles permit accommodation of the thermal expansion of the metal. Such an approach (the application of individual segments) would not be practical in the case of gas turbine engine components in which extreme operating conditions will be encountered and which a multiplicity of small complex parts must be coated at a reasonable cost. Additionally, in the use of such a segmented ceramic approach, there still remains the problem of obtaining a good metal-ceramic bond.
In a different art area, it is known to apply coatings of ceramics and metals by vapor deposition. The general subject of vapor deposition is described in an article by R. F. Bunshah "Journal of Vacuum Science of Technology," Vol. 11, No. 4, July/August 1974. The application of ceramics by vapor deposition is employed chiefly in the semiconductor and optics industries where extremely thin coatings are used.
In vapor deposition, the article to be coated is held over a molten pool of material of appropriate composition which evaporates, and the vapor condenses on and coats the article. This process is used in a variety of applications including the application of metallic coatings to gas turbine engine parts. The application to gas turbine engine parts is described in the "Journal of Vacuum Science of Technology," Vol. 11, No. 4, July/August 1974, pgs. 641 through 646 in an article by Boone et al.
This article also describes the types of defects which can occur in vapor deposited coatings. The most significant defect described is termed a "columnar defect" in which the coating forms as columnar grains which are poorly bonded to each other. Such a structure is described as being detrimental because the exposed columnar surface greatly increased the surface exposed to the environment and because the gaps between the columns may adversely affect mechanical properties. The article indicates that practical uses of vapor deposited coatings requires that the columnar type of structure be minimized.
A paper entitled "High Rate Sputtered Deposition of Protective Coatings on Marine Gas Turbine Hot Section Superalloys," authored by J. Fairbanks et al was presented in July 1974 at a conference on "Gas Turbine Materials in the Marine Environment" and was subsequently presented as a report by the Metals Information Center of the Department of Defense (MCIC 75-27). The paper indicates that columnar growth defects were observed at sputtered ceramic coatings. The paper hypothesizes that a coating with a columnar structure might permit stress relaxation of the coating and thereby enhance coating life.
Subsequent development of this concept by one of the authors is detailed in NASA Report NASA-CR-159412 issued July 19, 1978. This report describes the sputter deposition of zirconia based columnar coatings on copper substrates. The investigation was not successful in producing a coating which could withstand cycling between -196.degree. F. and 400.degree. F. The investigators also performed experiments in which a titanium interlayer was deposited prior to the ceramic deposition. However, the resultant coatings spalled under conditions of moderate thermal cycling. In the conclusion of the report, the investigator indicated that the coating performance was substantially less than that observed in previous work using graded coatings. The same investigator also performed work for the Naval Sea Systems Command on graded metal-ceramic coatings applied by sputtering in which columnar coatings were produced. These coatings were unsuccessful in resisting spalling under conditions of severe thermal cycling. The report is entitled "Develop Sputter Deposited Graded Metal ZrO.sub.2 Coating Technology for Application to Turbine Hot Section Components," Contract No. N00024-75-C-4333, Oct. 11, 1976.
Ceramic coatings have also been applied by a plasma spray process. The most successful plasma spray coatings to date have been applied to articles which have been previously coated with a metallic bond coat. Among the bond coats investigated have been the MCrAlY class of materials. In this situation, the bond coat appears to function by acting as a soft, rough layer in which the plasma spray particles are embedded forming a mechanical bond. This is described in U.S. Pat. No. 4,055,705 and pending application U.S. Ser. No. 811,807 (now U.S. Pat. No. 4,248,940) which pending application has been the prevailing application in Interference No. 100,011.